M-x mew with (setq debug-on-error t) gives:
Signaling: (wrong-type-argument stringp nil)
string-to-int(nil)
mew-update-range()
mew-input-range("+inbox")
mew-summary-get()
mew(nil)
call-interactively(mew)
execute-extended-command(nil)
* call-interactively(execute-extended-command)